1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus having a gradient coil system.
2. Description of the Prior Art
For generating images of the inside of the body of an examination subject, a magnetic resonance apparatus hasxe2x80x94among other thingsxe2x80x94a basic field magnet system for generating a static basic magnetic field as well as a gradient coil system for generating rapidly switched gradient fields. Currents with amplitudes reaching several 100 A and that are subject to frequent and rapid changes in the current direction, with rise and decay rates of several 100 kA/s, flow in gradient coils of the gradient coil system. In a basic magnetic field on the order of magnitude of 1 Tesla, these currents cause mechanical oscillations of the gradient coil system due to Lorentz forces.
These oscillations are transmitted to the entire surface of the magnetic resonance apparatus via various propagation paths. Dependent on the surface velocity of the surface regions, the mechanical oscillations of the various surface regions are transformed into acoustic oscillations that ultimately cause noise that is disturbing to patients and attending personnel.
For example, German OS 197 22 211 discloses a structure of a hollow-cylindrical gradient coil system with shielding coils. From inside to outside, the gradient coil system has the following elements, which are fashioned in hollow-cylindrical regions arranged concentrically relative to one another: a first transverse gradient coil, a second transverse gradient coil, a first cooling device, a longitudinal gradient coil, a shim assembly, a second cooling device, a longitudinal shielding coil, a first transverse shielding coil and a second transverse shielding coil. These elements are cast in one work sequence to form the gradient coil system. The cooling devices as described, for example, in German OS 197 21 985, corresponding to U.S. Pat. No. 6,111,412.
Further, a number of passive and active noise-reducing measures in magnetic resonance apparatus are known. Known passive noise-reduction measures include, for example, a attachment of sound-damping foamed materials in cladding parts toward the gradient coil system and/or an arrangement of flexible layers at and/or in the gradient coil system. U.S. Pat. No. 4,954,781 discloses an example of such a measure.
German PS 197 33 742, corresponding to U.S. Pat. No. 6,075,363, further, discloses a method for noise-reduction during operation of a gradient coil of a magnetic resonance apparatus wherein the gradient coil is at least partly in contact with a reaction resin molding material, and the reaction resin molding material is kept at a temperature during operation of the gradient coil that lies in the region of the glass transition temperature of the reaction resin molding material. Maintaining this temperature is accomplished, for example, by regulation of a cooling system connected to the gradient coil, for example a water cooling system.
An object of the present invention is to provide a magnetic resonance apparatus with an improved gradient coil system that, in particular, exhibits low noise emission values.
This object is inventively achieved in a gradient coil system of a magnetic resonance apparatus having at least one flexible and thermally conductive damping structure that contains a flexible matrix and thermally conductive, particulate fillers, with at least parts of a cooling device of the gradient coil system being arranged within the damping structure. The damping structure has a hysteresis loop in the force/deformation diagram whose encompassed area is so large that the damping structure absorbs much oscillation energy of the gradient coil system. Since the damping structure is arranged in the immediate spatial proximity of the gradient coils, oscillations of the gradient coils can be damped directly at the location that causes them. As a result, the damping effect is large and a propagation of oscillations over the entire apparatus is substantially prevented. By arranging at least parts of at least one cooling device of the gradient coil system within the flexible and thermally conductive damping structure, the cooling as well as the oscillation damping of the gradient coils are accomplished by a space-efficient combined assembly. The damping structure exhibits high thermal conductivity, so that an efficient elimination of heat that arises in the gradient coils during operation thereof is assured by the cooling device across the damping structure.